[ticket/11424] Handle race condition properly

In circumstances when quickmod-race occur, `General Error` message will
instead be handled based on action taken. On allowed quickmod tools,
display suitable error message with proper reasons.

PHPBB3-11424
[ticket/11424] Minor update

- Code changes based on guidelines mentioned for phpBB
coding/programming.
- Switch default case can be handled outside if statement.

PHPBB3-11424